The invention relates to a process for purifying zirconium tetrachloride and, more particularly, to a process for separating hafnium tetrachloride contaminants from zirconium tetrachloride for producing nuclear grade zirconium.
Zircon and other natural sources of zirconium typically contain from about 0.1% up to about 2.5% hafnium based upon the zirconium content. However, hafnium is considered a contaminant in nuclear grade zirconium and must be reduced to very low levels. Similarly, the specifications for nuclear grade hafnium limit the concentration of zirconium to very low levels. Unfortunately, zirconium and hafnium have similar properties and are very difficult to separate. In commercial processes for producing nuclear grade zirconium, zirconium oxide contaminated with hafnium oxide is chlorinated to produce a mixture of zirconium tetrachloride and hafnium tetrachloride. The mixture of tetrachlorides is then dissolved in an aqueous acid and the metals are separated in a liquid-liquid extraction operation using organic solvents such as methyl isobutyl ketone and the like. The separated zirconium may then be reduced to metallic form by reduction with magnesium in a Kroll Process. Although liquid-liquid extraction operations effectively separate these metals, these types of operations employ large quantities of organic solvents and other chemicals which results in ongoing environmental and operating problems. It would therefore be desirable to separate zirconium and hafnium by other means which would not employ organic solvents.
It was proposed as early as the 1950s to separate zirconium tetrachloride from hafnium tetrachloride by first reducing zirconium tetrachloride to zirconium trichloride with a reducing agent such as iron (Eo=0.771 volts), aluminum (Eo=1.662 volts) or zirconium dihalide and then separating the zirconium trichloride from the hafnium tetrachloride. See, e.g., U.S. Pat. Nos. 2,961,293; 2,953,433; 2,916,350; 2,885,281 and 2,791,485. However, later testing has shown that iron and aluminum undesirably tend to reduce the hafnium tetrachloride along with the zirconium tetrachloride so that the zirconium cannot be effectively purified. In addition, the solid-solid reactions between the zirconium tetrachloride on one hand and the iron or aluminum reductant on the other hand tend to be relatively inefficient. Further, iron and aluminum are considered to be contaminants in zirconium and its alloys and are closely controlled.
It has also been proposed to reduce zirconium tetrachloride with hydrogen (Eo=0 volts), but it has been found that hydrogen will not reduce zirconium tetrachloride unless atomic hydrogen is generated as a preliminary step.
Thus, the art has not developed a practical alternative to liquid-liquid extraction and like operations for purifying zirconium tetrachloride.